screw pitch
The pitch of metric standard teeth is as follows:
M 1.6*0.35
M2*0.4
M2.5*0.45
M3*0.5
M4*0.4
M5*0.8
M6* 1.0
M8* 1.25
M 10* 1.5
M 12* 1.75
M 14*2.0
M 16*2.0
M 18*2.5
M20*2.5
M22*2.5
M24*3.0
M27*3.0
M30*3.5
M33*3.5
M36*4.0
Brief introduction of turning thread
The method of turning the surface of workpiece into thread is called turning thread. Threads are divided into triangular threads, trapezoidal threads and square threads according to tooth types. Among them, the commonly used metric triangular thread is the most widely used.
Thread type
1. Basic tooth profile of common triangular thread
The basic tooth profile of ordinary triangular thread is shown in Figure 2, and the basic size names are as follows:
Basic tooth profile of ordinary triangular thread
D refers to the major diameter (nominal diameter) of the internal thread;
D- major diameter of external thread (nominal diameter);
D2—— internal thread middle diameter;
D2—— pitch diameter of external thread;
D 1- internal thread diameter;
D 1- external thread diameter;
p-pitch；
The height of the original triangle.
There are three basic elements that determine threads:
Tooth profile angle α The angle between two sides of the internal thread in the axial section of the thread. Metric thread α=60o, English thread α=55o.
Pitch p is the distance between corresponding points in the axial direction between two adjacent teeth.
Thread diameter D2(d2) It is the diameter of an imaginary cylinder, and its theoretical height is H of a flat thread. The thread thickness is equal to the groove width at the middle diameter. Only when the internal and external threads have the same diameter can they be well matched.
2. Methods and steps of turning external thread
(1) preparatory work
1) When installing a threaded turning tool, the tip angle of the turning tool is equal to the thread profile angle α=60o, and its rake angle γo=0o can ensure the thread profile angle of the workpiece, otherwise the profile angle will produce errors. Only when rough machining or thread precision is not high, the rake angle can be γ o = 5o ~ 20o. When installing the thread turning tool, the tool tip should be aligned with the center of the workpiece and the tool should be aligned with the template, so that the bisector of the tool tip angle can be ensured to be perpendicular to the axis of the workpiece, so that the turned tooth angle will not deviate.
Figure 3 Geometric angle of thread turning tool and alignment between tool and template
2) Turn the thread excircle according to the thread specification, and carve the thread length termination line according to the required length. First, the outer diameter of the thread is turned to size, and then a slightly visible line is carved on the thread end of the workpiece with the tip of the knife as the tool withdrawal trace of the turned thread.
3) According to the pitch p of the workpiece, check the marks on the machine tool, then adjust the position of the handle on the feed box and change the number of teeth of the gear of the change gear box to get the required pitch of the workpiece.
4) Determine the spindle speed. Novices should adjust the spindle speed of the lathe to the lowest.
(2) threading methods and steps
1) Determine the starting position of thread cutting depth, adjust the scale of the middle slide plate to zero, start the trolley, make the tip of the knife touch the surface of the workpiece slightly, and then quickly adjust the scale of the middle slide plate to zero to facilitate feed counting.
2) Try to cut the first spiral and check the pitch. Shake the tool rest to 8~ 10 teeth away from the end face of the workpiece, and the transverse feed is about 0.05. Start the trolley, close the opening and closing nut, turn a spiral on the surface of the workpiece, exit the turning tool at the thread termination line, open the reverse handle and retreat the turning tool to the right end of the workpiece; Stop the car and check whether the pitch is correct with a steel ruler.
3) Adjust the reverse feed with the dial and start cutting, as shown in Figure 4d. According to the empirical formula ap ≈ 0.65p, the relationship between total thread backbiting ap and pitch is about 0. 1.
4) When the turning tool is near the finish line, you should be prepared to stop the turning tool, first exit the turning tool quickly, and then drive the car out of the tool rest backwards. As shown in fig. 4e.
5) Feed horizontally again and continue cutting until the correct tooth profile is turned out, as shown in Figure 4f.
Thread cutting method and steps
3. Matters needing attention when turning threads
1) Pay attention to and eliminate the "idle travel" of the car.
2) Avoid "disorderly buckle". When the first spiral is turned and the second feed is turned, the tip is not on the original spiral (spiral pile), but to the left or right, even in the middle of the tooth tip. The phenomenon of screwing is called "disorderly screwing", and the way to prevent screwing is to use the reverse (positive and negative) rotation method. When turning threads by left and right corner cutting method, the moving distance of the small carriage should not be too large. If the tool is damaged during turning and needs to be replaced again or the opening and closing nut is inadvertently lifted, attention should be paid to timely tool alignment.
3) Tool alignment: Before tool alignment, install the threaded lathe, then press the opening and closing nut, start the straight lathe (be careful not to idle) and stop it, move the middle and small carriage so that the tip of the knife accurately falls into the original spiral groove (be careful not to move the big carriage), mark the feed of the middle carriage again according to the position in the spiral groove, then withdraw the lathe and drive it back to the threaded head before entering. Be sure to pay attention to the car on the right when aiming at the knife.
4) Power-assisted tool: Power-assisted tool means that after the thread is turned to a certain depth, the small carriage moves forward or backward for turning. When borrowing tools, be careful that the moving distance of the car should not be too large, so as not to cause the "disorderly buckle" of the width of the tank car.
5) When using the clamping method of two thimbles to turn threads, when the workpiece is unloaded and then turned, the tool should be aligned before turning to avoid "disorderly turning threads".
6) Safety precautions:
(1) Before threading, check whether all handles are in the threading position to prevent blind driving.
(2) Thread turning should be focused, fast and sensitive; \'
(3) When turning threads with high-speed steel turning tools, the speed of headstock should not be too fast to avoid tool wear;
(4) Prevent the lathe tool, tool rest and carriage from colliding with chuck and bed end;
(5) When screwing the nut, the turning tool should be withdrawn from the workpiece to prevent the turning tool from cutting hands. Do not drive to tighten or withdraw the nut;
(6) The rotating thread cannot be touched by hand or wiped with cotton yarn.
4. Quality analysis of lathe external thread
Causes and preventive methods of waste products in thread turning
Calculation of thread diameter
First of all, you need to know the detection of threads. Bottom diameter is not the main dimension. Pitch diameter measurement for threads. Of course, the base diameter is a free tolerance.
Generally, when cutting external threads, the base diameter is equal to the nominal diameter minus the product of the pitch and 1.3. This 1.3 is a constant and needs to be remembered.
Secondly, it depends on the obtuse angle or arc radius of the tip of the thread cutter you grind, that is to say, the cutting depth of 2.4 mentioned in the book is actually an example, for reference only, not an accurate value. When cutting threads, measure the middle diameter with a thread micrometer, or clamp steel needles at both ends of the thread and measure with an outside micrometer, but the tolerance needs to be calculated according to the diameter of the steel needle and the thread angle. Generally, it is measured with a threaded micrometer or something that does not need a close fit.
Example above: base diameter =30-2* 1.3=27.4.
And the thread diameter needs to be reduced by pitch * 10%=0.2.
And the cutting depth = outside diameter-bottom diameter =(30-0.2)-27.4=2.4.
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screw thread
Snail species;
According to the different uses of the line, it can be divided into:
1. International metric thread system:
Threads used in Chinese national standards. The tooth top is flat and easy to turn, while the tooth bottom is flat.
Circular arc increases thread strength. The thread angle is 60 degrees, and the specification is expressed by m.public.
Threads can be divided into coarse teeth and fine teeth. Expressed as M8x 1.25. (male: code name, 8: male)
Scale diameter, 1.25: pitch).
2. American standard thread:
The top and root of the thread are flat and have good strength. The thread angle is also 60 degrees, and the specifications are as follows
How many teeth per inch? This thread can be divided into coarse teeth (NC); Fine teeth (nf); Very fine tooth
(NEF) level 3. Expressed as 1/2- 10NC. (1/2: external diameter; 10: number of teeth per inch; network computer
Code name).
3. Unified thread:
Developed by the United States, Britain and Canada, it is a commonly used English thread at present.
The thread angle is also 60 degrees, and the specification is expressed by how many teeth per inch. This thread can be divided into coarse teeth.
(United Nations Command); Fine teeth (UNF); Very fine teeth (UNEF). Expressed as 1/2- 10UNC. ( 1/2:
Outer diameter; 10: number of teeth per inch; UNC code)
4. Sharp V-shaped thread:
The top and roots are sharp, weak and not commonly used. The thread angle is 60 degrees.
5. whitworth thread:
Threads adopted by British national standards. The thread angle is 55 degrees and the symbol is "W".
Suitable for rolling manufacturing. The expression is as follows: W 1/2- 10. (1/2: external diameter; 10: number of teeth per inch;
W code).
6. Round thread:
German DIN standard thread. Suitable for the connection of light bulb and rubber tube. express
The symbol is "Rd".
7. Pipe thread:
Threads used to prevent leakage are usually used to connect gas or liquid pipe fittings. Thread section angle
55 degrees, divided into straight pipe thread code "P.S., N.P.S" and inclined pipe thread code "two kinds.
Noun (abbreviation for noun) Its taper is 1: 16, that is, 3/4 inches per foot.
8. Square thread:
The transmission efficiency is high, second only to ball thread, but it cannot be adjusted by nut after wear.
Disadvantages. Generally used for vice screws and crane threads.
9. Trapezoidal thread:
Also known as Acme thread. The transmission efficiency is slightly less than that of square thread, but it can be adjusted by nut after wear.
The whole. The metric thread angle is 30 degrees, and the English thread angle is 29 degrees. Generally used in lathes.
Lead screw The symbol is "Tr".
10. Thread:
Also known as oblique square thread, it is only suitable for one-way transmission. Such as screw jack, pressurizer, etc.
The symbol is "no"
1 1. Ball thread:
As the thread with the best transmission efficiency, it is difficult to manufacture and the cost is extremely high, so it is used for precision.
Mechanically. Such as the lead screw of a numerical control machine tool.
Representation of inch bolts
LH 2N 5/8 × 3 - 13 UNC━ 2A
(1)LH is the left thread (RH is the right thread, which can be omitted).
(2)2N double threads.
(3) 5/8 inch thread with an outside diameter of 5/8 inch.
(4) The length of three bolts is 3 inches.
(5) 13 thread has 13 teeth per inch.
(6)UNC unified standard thread coarse teeth.
(7) Two-stage fit, external thread (3: tight fit; 2. China cooperation; 1: loose fit) a: external thread
Thread (can be omitted) b: internal thread
Inch thread:
The size of English thread is usually expressed by the number of threads per inch of thread length.
It is called "teeth per inch", which is exactly equal to the reciprocal of the pitch. For example, 8 threads per inch,
Its spacing is 1/8 inches.
Thread processing method
There are two main methods to process internal and external threads on the workpiece: cutting and rolling.
The application of the thread principle can be traced back to the spiral water lifting tool created by the Greek scholar Archimedes in 220 BC. In the 4th century A.D., Mediterranean countries began to apply the principle of bolts and nuts to wine-making presses. At that time, the external threads were all wrapped around a cylindrical rod with a rope and then carved according to this mark. The internal thread is usually formed by hammering with soft material wrapped around the external thread. /kloc-about 0/500, Leonardo, Italy? In the sketch of thread processing device drawn by Leonardo da Vinci, there is an idea of processing threads with different pitches with female screws and exchange gears. Since then, the method of mechanical thread cutting has been developed in European watch manufacturing industry. 1760, British brothers J. Wyatt and W. Wyatt obtained a patent for cutting wooden screws with a special device. 1778, an Englishman J. ramsden made a thread cutting device driven by a worm gear pair, which can process long threads with high precision. 1797, an Englishman Maudslay H. turned metal threads with different pitches by using mother screw and exchange gear on his improved lathe, which laid the basic method for turning threads. 1In the 1920s, Maudslay manufactured the first batch of taps and dies for thread processing. At the beginning of the 20th century, the development of automobile industry further promoted the standardization of thread and the development of various precise and efficient thread processing methods. Various automatic die opening heads and automatic tap shrinking were invented one after another, and thread milling began to be applied. In the early 1930s, thread grinding appeared. Although the wire rolling technology was patented at the beginning of 19 century, it developed slowly because of the difficulty in mold manufacturing. It was not until World War II (1942~ 1945) that the precision problem of die manufacturing was solved due to the need of munitions production and the development of thread grinding technology.
1) thread cutting generally refers to the method of machining threads on workpieces with forming tools or abrasives, mainly including turning, milling, tapping, tapping, grinding, grinding and cyclone cutting. When turning, milling and grinding threads, the transmission chain of the machine tool ensures that the turning tool, milling cutter or grinding wheel moves a lead accurately and evenly along the axial direction of the workpiece every time the workpiece rotates. When tapping or tapping, the cutter (tap or die) moves relative to the workpiece, and the cutter (or workpiece) is guided by the pre-formed thread groove for axial movement.
Forming turning tool or thread combing tool can be used to turn threads on lathe (see thread processing tool). Because of its simple structure, forming turning tool is a common method to produce threaded workpieces in small batches. Using thread comb cutter to turn threads has high production efficiency, but the cutter structure is complex, which is only suitable for turning fine teeth and short threads in medium and large-scale production. Generally, the pitch accuracy of trapezoidal thread turning by ordinary lathe can only reach 8~9 grades (JB2886-8 1, the same below); Machining threads on a special thread lathe can significantly improve productivity or accuracy.
2) Thread milling
Milling with disc cutter or comb cutter on thread milling machine. Disc milling cutter is mainly used for milling trapezoidal external thread on screw, worm and other workpieces. Comb-tooth milling cutter is used for milling internal thread and taper thread. The length of the working part of multi-edge milling cutter is longer than the length of the thread to be machined, so the workpiece can be machined only by rotating 1.25~ 1.5, and the productivity is very high. The pitch accuracy of thread milling can generally reach 8~9 grades, and the surface roughness is R5~0.63 microns. This method is suitable for mass production of threads with average precision or rough machining before grinding.
3) Thread grinding is mainly used to process the precision thread of hardened workpiece on the thread grinder.
Thread grinding can be divided into single-line grinding wheel and multi-line grinding wheel according to the different cross-sectional shapes of grinding wheels. Single-wire grinding wheel can achieve pitch accuracy of 5 ~ 6 and surface roughness of R 1.25~0.08 micron, which is convenient for grinding wheel dressing. This method is suitable for grinding precision screws, thread gauges, worms, small batches of threaded workpieces and shovel grinding precision hobs. Multi-line grinding wheel grinding is divided into longitudinal grinding method and cut-in grinding method. In the longitudinal grinding method, the width of the grinding wheel is smaller than the length of the grinding thread. By moving the grinding wheel longitudinally for one or several strokes, the grinding wheel can be reversely tapped to the final size. The width of grinding wheel in plunge grinding method is greater than the length of grinding thread. When the grinding wheel cuts into the surface of the workpiece radially, the workpiece can be ground in about 65438 0.25 revolutions, with high productivity, but slightly lower accuracy, and the grinding wheel dressing is more complicated. The cut-in grinding method is suitable for large-scale shovel grinding of taps and grinding of some fastening threads.
4) Thread grinding Nut-type or screw-type thread grinding tools are made of soft materials such as cast iron, and the parts of the workpiece with pitch errors are ground by forward and backward rotation to improve the pitch accuracy. Hardened internal threads are usually ground to eliminate variations and improve accuracy.
5) Tapping and tapping
Screw the tap into the pre-drilled bottom hole on the workpiece with a certain torque to process the internal thread.
Turning thread is to cut the external thread on the rod (or tube) workpiece with a mold. The machining accuracy of tapping or tapping depends on the accuracy of tap or die. Although there are many ways to process internal and external threads, small-diameter internal threads can only be processed with taps. Tapping and tapping can be done by hand, or by lathe, drilling machine, tapping machine and tapping machine.
6) thread rolling: the workpiece is plastically deformed by a forming and rolling die to obtain threads. Wire rolling is generally carried out on a wire rolling machine. The thread rolling machine is carried out on an automatic lathe with an automatic thread rolling head, which is suitable for mass production of external threads of standard fasteners and other threaded connectors. Generally, the outer diameter of rolling thread is not more than 25mm, the length is not more than 100mm, the thread accuracy can reach Grade 2 (GB 197-63), and the diameters of all blanks are roughly equal to the middle diameter of the processed thread. Generally, the internal thread cannot be processed by rolling, but for soft workpieces (the maximum diameter can reach about 30 mm), the internal thread can be cold extruded by slotless extrusion tap, and the working principle is similar to tapping. The torque required for cold extrusion of internal thread is about 1 times that required for tapping, and the machining accuracy and surface quality are slightly higher than tapping.
The advantages of wire rolling are: the surface roughness is less than that of turning, milling and grinding; Due to cold working hardening, the strength and hardness of the rolled thread surface can be improved; High material utilization rate; The productivity is twice as high as that of cutting, and it is easy to realize automation; The rolling die has long service life. However, rolling thread requires that the hardness of workpiece material should not exceed HRC40；; It requires high dimensional accuracy of the blank; The precision and hardness of the rolling die are also high, and it is difficult to manufacture the die; Not suitable for asymmetrical thread of hobbing.
According to the different rolling dies, rolling threads can be divided into thread rolling and thread rolling.
Two thread-shaped wire rolling plates are arranged oppositely, and the staggered spacing is 1/2. The static plate is fixed, and the moving plate does reciprocating linear motion parallel to the static plate. When a workpiece is fed between two plates, the moving plate moves forward to rub and press the workpiece, so that its surface is plastically deformed into a thread.
There are three kinds of thread rolling: radial thread rolling, tangential thread rolling and head thread rolling. Radial thread rolling: two (or three) thread rolling wheels are installed on parallel shafts, and the workpiece is placed on the bracket between the two wheels, and the two wheels rotate in the same direction and at the same speed.
One of the wheels also moves radially. The workpiece is driven by the threaded roller to rotate, and the surface is radially extruded to form threads. For some lead screws with low precision requirements, a similar method can also be used for roll forming. Tangential wire rolling: also known as planetary wire rolling, the wire rolling tool consists of 1 rotating center wire rolling wheel and 3 fixed arc wire plates.
During wire rolling, the workpiece can be continuously fed, so the productivity is higher than that of wire rolling and radial wire rolling. Thread rolling: it is carried out on an automatic lathe and is generally used to process short threads on workpieces. The rolling head has 3~4 wire rollers evenly distributed around the workpiece.
During wire rolling, the workpiece rotates, and the rolling head feeds axially to roll the workpiece into threads.